Parallel circuit arrangement for power tubes



Feb. 1% 1950 A H. GOLDBERG ETAL 2,497,166

PARALLEL CIRCUIT ARRANGEMENT FOR POWER TUBES Filed June 6, 1944 2 Sheets-Sheet 1 I N V EN TOR. HHROLD GOLDBERG 1 1 EDWHRD/FDE MERS Feb. M, 1950 H. GOLDBERG ET AL 2,497,166

\ PARALLEL CIRCUIT ARRANGEMENT FOR POWER TUBES 2 Sheets-Sheet 2 Filed June 6, 1944 INVENTOR.

Patented Feb. 14, 1950 PARALLEL CIRCUIT ARRANGEMENT FOR POWER TUBES Harold Goldberg, Irondequoit, and Edward F.

De Mers, Rochester, N. Y., assignors to Stromberg-Carlson Company, Rochester, N. Y., a corporation of New York Application June 6, 1944, Serial No. 538,920

'7 Claims. (Cl. 315-195) This invention relates to circuit networks in which are discharge devices are operated in parallel, multi-parallel or series-parallel relation in pulse generators.

' In certain cases, it is desirable to develop a greater power output than can be provided by a single arc discharge device. However, it has been necessary in the past to use equalizing resistors with their attendant power losses where such arc discharge devices were operated in parallel. Such equalizing resistors have been required since supposedly identical devices have slightly different breakdown characteristics. Consequently, when one of the devices became conducting the anode potential of the second device would never be sufiicient to establish current flow therethrough in the absence of some equalizing means.

In accordance with the main feature of the present invention, there are provided circuit networks for operating two or more are discharge devices in parallel, in multi-parallel or in seriesparallel in a pair of simple or complex branches without the need for equalizing resistors or other material power-dissipating control units. In the circuit arrangement of the present invention means common to a given pair of branches of such parallel-connected devices, is effective when one branch is conducting, for raising the voltages in the other branch thereof until it also becomes conducting. After both branches become conducting; the current division between them may be apportioned according to any desired ratio.

Another feature of this invention relates to a circuit arrangement in which dissimilar types of arc discharge devices can be operated in parallel.

A further feature of the invention relates to the pyramiding of parallel circuits for operating arc discharge devices in multiple. By this pyramiding of the parallel circuits, 2. theoretically large number of circuit combinations can be made, the limit on this number being determined by the complexity of the resulting networks which may render them impracticable for use.

In the drawings Fig. 1 is a diagrammatic showing for the parallel operation of similar are discharge devices from a direct current source in a pulse generator;

Fig. 2 is a portion of a modified parallel circuit arrangement wherein a Thyratron is utilized in one parallel branch and a spark gap is used in the other parallel branch;

Fig. 3 is a fragmentary showing of a modified series-parallel arrangement wherein two are discharge devices are connected in series in each parallel branch;

Fig. 4 is a diagrammatic showing of a portion of a modified parallel circuit arrangement in which the parallel circuits are pyramided;

Fig. 5 is a diagrammatic showing of a portion of a modified form of the invention in which the parallel circuits are further pyramided; and

Fig. 6 is a diagrammatic showing of a portion of a circuit arrangement in which a single arc discharge device is connected in parallel with a branch consisting of two parallel branches, in each of which there is included an arc discharge device;

Fig. 7 is a diagrammatic showing of a modified form of the invention in which the Thyratrons are replaced by spark gaps.

In Fig. 1 of the drawings there is illustrated a circuit arrangement of this invention wherein there are utilized two electrostatically controlled arc discharge devices 5 and 5a commonly referred to as grid glow tubes or Thyratrons. The device 5 comprises an anode 6, at least one grid 1 and a cathode 8 enclosed in an envelope containing ionizable gas. The device 5a is identical in construction with device 5 and the corresponding parts of the two devices are identified by the same numerals, with the suffix a added in the case of the parts of device 5a, The cathodes 8 and 8a are connected in parallel by the conductor 22 to one side of the direct current source D. C. These cathodes are also connected by conductor 22 to one terminal of the load 23. This load may be a resistance or a transformer coupled load. The terminals l6 and I! of the transformer l2 are also connected by the conductor 24 and the impedance 25 in series therewith, which may be a resistance or an inductance, to the second terminal of the direct current source. The conductor 24 also leads to one terminal of the network N, the other terminal of which is connected by lead 26 to the remaining terminal of the load 23. The network N, which may have various forms, is herein shown as comprising capacitors 21and inductors 28. The input circuit of the tubes comprising the cath-' odes 8, 3a and the control grids 1 and lot, has connected therein, a source of triggering potential of the character indicated by the graph enclosed within the rectangle S.

During operation, the capacitors 21 of the network N are charged through the-impedance 25, from the source D. C. When, however, a positive triggering pulse from the'source Sis applied to the grids I andl'a to render the tubes 5 and 5a,

conducting, the network N will discharge through these tubes into the load 23. The windings H and !5 of the transformer 12 respectively connected in the anode circuits of these tubes insure simultaneous operation of the tubes in the following manner.

The transformer l2 must have the following characteristics. First, the coupling between both of its windings must be extremely good, as good, in fact, as the state of the art will permit. The degree of coupling required will of course depend on the time duration of the pulse to be generated but in general it should be madeas good as pos-- sible taking into account other required charac teristi-cs of the transformer. In general, the coupling requirements necessitate the use of iron cores of the very best characteristics. Second, the polarity of the windings should be as indicated in Fig. 1. Third, the transformer should be designed so that saturation does not take place under conditions of operation. Fourth, the selfinductance of the windings should be made as small as possible consistent with the above conditions. Fifth, the turns ratio between the two windings should be set according to the ratio of current division-desired between the two branches. The manner in which the current is divided will bedescribed.

Let it be assumed that the triggering pulse has been applied to the tubes5 and 5a but that due toisome dissimilarity in these tubes, only tube 5 is caused to conduct. In-the ordinary case, where the two devices have their plates in parallel, the

drop in potential due to one tube firing insures that the tube in paralleltherewith will never fire. In accordance with the present invention, however, the efiect of one tube firing is to place the voltage E to which the network has been charged, across winding H. A transient response will occur which will cause a voltage to appear on winding l5- with the polarity indicated. If the turns ratio of the transformer is unity, this voltage will rise toa value of roughly E with the polarity shown and in conjunction with the network, will result in a momentary-voltage 2E being applied tothe tube 5a. The time for this to take placedepends primarilyon the self-inductance of the transformer windings, the coupling, and the stray capacitance of the circuit. With verytight coupling. it canbe made to occur, in certain embodiments of the invention, in a fraction of a microsecond. Under this condition, tube 5a must also fire. Thus, the firing ofboth of' these tubes is assured.

With respect to current division, very slight differences in the characterist cs of two are dis-. charge tubes connected in parallel, will ordinarily cause, gross differenc s in the current carried by the two. The devices are usually placed in. parallel because the safe current handling capacity of one tube may be insuflicient for the purpose at hand. If. however, the current division in parallel is so poor that one tube has its current handling capacity exceeded; nothing is ga ned by paralleling them. In accordance with this invention, the transformer l2. in addit on to insuring firing of both tubes, also insures the proper current divi ion. This is best ex lained by considering aga n the case in which the turns ratio of transformer I2 is unity. There are two cases: first, whereboth tubes fire simultaneously in which case both initial currents are equal and small and second, where the tubes fire consecutively in which case both initial currents are small but the initial current inthe initiallyfired;

tube is very slightly larger than current in the later fired tube. In the latter case, the current cannot rise to an appreciable value in the tube first to fire because in the absence of the conduction of the second tube the self-inductance of the transformer winding. will limit this current. Therefore, in either case, at the instant of firing, the two tubes are conducting negligible current. The current will now build up to its peak value in the two tubes. If one tries to rise faster than the other, the transformer I2 will prevent this since voltages will be induced which slow the rise in the tube attempting to take more of the current and, increase the rise in the tube which is lagging. Since, itz'takes very little voltage to change the currentin a discharge tube, the resultis that the buildup of current in the tubes is practically identical. Furthermore, after the current has built up to its maximum value, the transformer will prevent, in essence, the departure of the'eurrent values from the values obtained-just after buildup. Therefore, the current is equally divided in the case of a turnsratio of unity. The tight coupling in the-transformer and the practically identical buildup time prevents the transformer from presenting too much inductance-to the discharge of thepulse network This is necessary. The inductive effect will be least when thecoupling is the tightest and the self-inductances, of the windings are reduced to their minimum valueconsistent with proper circuit performance.

Sometimes, it is desirable to-operate in arallel twotubes ofunequal current handling capabili ties and to-provide means for dividing the currentsaccording to their ratings. This may bedone by making the turns ratio of the trans former [2, equal to the desired current ratio. All other characteristics of thetransfor-mer such as tight coupling, etc., must also be preserved. The tube which is to carry thegreater currentmust beconnected to the winding having the smaller number of turns. In thecase of starting, if one tube only fires, the following maximum voltages may be presented tov the unfired tube. If. the low current tube fires, the voltage possibly presented to the unfired, tube is E plus ZE/nv where n. is the turns, ratio of T. If the high current tube fires first, thenthe voltage may rise to E plus 2En on the unfired tube. Finally, if the current buildup is such that the time rate of: change of the currents in the two windings is in the ratio 11., then the final peak currents are in the ratio 11..

In the form of the invention just described, electrostatically controlled arc, discharge devices or *Thyratrons have beenused. However, dissimilar types of discharge devices, such as: a Thyratron 30, in one parallel branch and a gasfilled spark gap 3| in the other parallel branch, can be used when the portion of thecircuit illustrated in Fig. 2 is substituted for that portion of the diagram of Fig. 1 which is enclosed by the dotted rectangle. In the resultingcircuit arrangement, the cathode 33 of the Thyratron 30 and the electrode 34 of the spark gap 3| are connected by conductor 22 to one side of the current source. Likewise, the anode 35' of the Thyratron and the electrode 35 of the spark gap are respectively connected through the windings 3? and 38 of an iron core transformer and conductor 24, to the other: side ofthe current source.

Fig. 3- when substituted for that portion of the circuit arrangement of, Fig, 1 which is enclosed,

within the broken line rectangle, indicates how two are discharge devices can be connected'and operated in series in each of the parallel branches of the network to provide a series-parallel arrangement. In this arrangement, the Thyratron 46 has its cathode 4| connected to the conductor 22 while its anode 43 is directly connected to the electrode 44 of the gas-filled spark gap or are discharge device 45. The other electrode 46 of this gap is connected in series with the winding 41 of the transformer 48 to the conductor 24. A resistor comprising sections 49a and 491), has one of its ends connected to the conductor 22 and its other end connected to the lower terminal of the transformer winding 41. An intermediate point on the conductor between the anode of the Thyratron and the electrode of the gap is connected to the common terminal of said resistor section, thereby serving as a voltage divider. The voltage of the network normally applied across the Thyratron and the spark gap is insuflicient to cause these devices to break down. However, when positive voltage is applied to the grid 4| or the Thyratron, the Thyratron becomes conducting. Under this condition, the cathode-anode drop of the Thyratron is reduced to a very low value, with the result that the voltage applied across the electrodes of the spark gap 45 is increased to the value where the spark gap breaks down and becomes conducting in series with Thyratron.

The current flowing through the Thyratron and spark gap in series, as well as through the transformer winding 41, will induce a voltage in the transformer winding 56 sufficient to cause current to flow in the other parallel branch. This parallel branch has a gas-filled spark gap 5| with its electrodes 52 and 53 connected in series with the gas-filled spark gap 54 which is provided with electrodes 55 and 56. Electrode 56 is connected to conductor 22. A resistor comprising sections 51a and 51b, is connected from the conductor 22 to the lower terminal of the transformer winding 50, an intermediate tap on this resistor being connected to the electrodes 53 and 55. The resistor sections serve to divide the voltage applied across the spark gaps 5| and 54 to render them conducting in response to the mentioned voltage induced in transformer winding 50. The manner in which a Thyratron, such as 40, and a spark gap, such as '45, connected in series, are rendered conducting, is more fully disclosed in the application of Goldberg et al., Serial No. 521,462, filed February 7, 1944, now abandoned.

The arrangements thus far described, disclose simple parallel arrangements and a series arrangement. However, the invention is applicable to a so-called multi-parallel arrangement, which results when the circuit starting network of Fig. 4 is substituted for that portion of the diagram of Fig. 1 which is enclosed within the broken line rectangle. In this modification, there are provided two parallel branches respectively designated A and B and connected across the conductors 22 and 24 connected to the respective sides of the current source. An iron-core transformer T has one of its windings 60 connected in series with branch A and its other winding 6| connected in series in branch B. Branch A is divided into two branches Al, A! respectively including the windings 62 and 63 of an iron core transformer 64. The winding 62 is connected in series with anode 65 of a Thyratron or other are discharge device 66. The cathode 61 of this device is connected to conductor 22. The otherwinding 63 of this transformer is connected to one of the electrodes 69 of a spark gap 10, which has its other electrode H connected to conductor 22.

Similarly, the branch B is divided into two parallel branches BI, Bl which respectively include the windings l4 and 15 of transformer 16. Spark gap 11 is connected to conductor 22 and is also connected in series with the transformer winding 14 while a second'spark gap 18 is likewise connected to conductor 22 and connected in series with the transformer winding 15.

In this arrangement, when the grid 68 of the Thyratron 66 has positive voltage applied thereto, the Thyratron will become conducting and as current flows therethrough, and through its related transformer winding 62, voltage is induced in the other winding 63 of the transformer which causes the spark gap 10 to break down.

Current now flowing in the main branch A and its transformer winding 66 induces voltage in.

the transformer winding 6| included in the main branch B. This voltage is suflicient to cause one or both of the gas-fi1led spark gaps 11 and 18 to break down and become conducting. In the event that only one of these spark gaps becomes conducting current flowing through its related winding of transformer 16 will induce voltage in the other transformer winding sufficient to cause its related spark gap to become parallel circuits for operating arc discharge de-' vices, are further pyramided. In this showing, the main branches A and B are inductively coupled together by a transformer and are respectively provided with intermediate branches Al, Al and BI, BI, as in Fig. 5. The intermediate branches Al, Al are coupled together by a transformer 8| and the intermediate branches Bl, Bl are coupled together by a transformer However, each branch Al is provided with a pair of sub-branches A2, A2, the respective pairs of sub-branches being coupled by the transformers 83 and 84.

anodes of Thyratrons 85 and 86 which have their cathodes connected to conductor 22. The windings of the transformer 84 are respectively connected in series with the spark gaps 88 and 88 to conductors 22.

Likewise, the main branch B is provided with intermediate branches Bl, Bl coupled together by transformer 82. is provided with a pair of sub-branches B2, B2, each pair being coupled together by transformers such as those designated 96 and 9!.

92, 93, 94 and 95 to the conductor 22.

In the operation of this arrangement, as in the case of those described above, positive voltage is applied to the grids 96 and 91 of the Thyratron:

The network" The windings of the, transformer 83 are respectively connected to the Each branch Bl, in turn,

The winding of the transformers 9B and 9! are re-' spectively connected in series with spark'gaps 7 devices. One of these will become-conducting, and'it'will insure operation of the Thyratron' in its related branch. This results in the break down of all are discharge devices in the remaining sub-branches in a manner similar to that set forth in the description of Fig. 4.

A further modified form of the invention results when the network of Fig. 6 is substituted for that portion of Fig. l included within the broken line rectangle. In this modification, the branch A includes a single arc discharge device such as a Thyratron Hill which is parallel with the branch B. Branch B has parallel subbranches including the arc discharge devices. The branches A and B are coupled together by the iron core transformer having the windings H13 and H14. The sub-branches are likewise coupled together by the windings Hi and Hit of an iron core transformer. The operation of this modification is similar to that of the modifications already described. Where one parallel branch is formed of two gaps in parallel, as disclosed in Fig. 6, it will be understood that if only one gap breaks down at the overvoltaging due to the Thyratron, the second gap will break down due to the overvoltage produced by the gap that has already fired. This embodiment of the invention is especially important in that the triggering of a spark gap or gaps may be controlled by the triggering of a Thyratron, a tube which is relatively easy to trigger.- Thus, the spark gaps may be employed to handle most of the power and the Thyratron may be used principally to trigger the system. The spark gaps involved in any system may be triggered, or caused to conduct, only by overvoltaging them, that is, by-applying, at least momentarily, a voltage in excess of the voltage that the gap can stand oiT. Likewise, in any pulse generator where the spark gaps are utilized, as herein disclosed, these gaps are used in such a manner that the breakdown voltage of the gap (or gaps) is always somewhat greater than the voltage to which the network is charged. Accordingly, the gaps must be overvoltaged to be fired. When being used in conjunction with a Thyratron, two results must be obtained. First, the firing or triggering of the Thyratron must-result in the firing of the gap. Second, the current must be made todivide between the gap and the Thyratron in the desired fashion. Let it be assumed that the ratio of turns of winding 31 to turns ofwinding 33 is n (Fig. 2). Under this assumption, when the Thyratron is fired, .as already described. the voltage of the network appears across winding 37. and a. voltage ZE/n, is. generatedin winding 38 with the polarity as shown. This occurs in a fraction of a microsecond. Therefore, a voltage E plus 57/12 is placed on the gap 3|. As long as this voltage E plus ZE/n is suflicient to fire the gap, it will fire and conduct and the Thyratron and gap will divide the current in the manner already described for two Thyratrons. The only limitation on'n is that E plus ZE/n must be sufficient to break down the gap.

All of the arrangements herein disclosed, have included at least one Thyratron for ease in iniating conduction throughout a given network. However, the invention is not limited in this manner since the Thyratrons can be replaced by spark gaps, as shown in Fig. 7. Of course, when a; spark gap device is used, conduction. therethrough is initiated by increasing the difierence .01 potential. applied across its electrodes. The

8 principles of my invention include the use of discharge devices having two or more electrodes, one serving as an anode and another serving as a cathode, the anode being that electrode which is connected to the more positive side of the starting circuit. Hence, when one device is rendered conducting, the voltage at the electrode serving as the anode of any other device paralleled with the conducting device is made more positive. No matter how many electrodes are employed, the non-firing or non-conducting discharge devices are fired or rendered conducting only by reason of the increase in anode potential.

'It should be emphasized that only a few of the typical modifications have been illustrated, but the invention includes various permutation, combinations and pyramiding of the several networks disclosed.

While the grid-glow tubes or 'Ihyratrons" are herein illustrated as being of the single grid construction, the invention is not so limited, since known multi-grid tubes of this type may also be used instead. It will also be understood that there can be many other variations and changes in the present disclosure without departing from the spirit.

What we claim is:

1. In a circuit arrangement of the class described, a source oi. direct current, a pair ofarcdischarge devices of unequal current handling ratings, each of said devices including a plurality of electrodes, and having certain corresponding electrodes thereof connected in parallel to said source, means for applying a control voltage to one of said devices for initiating the flow of current therethrough, and means responsive to the flow of current through said one device for increasing the voltage applied to the more positive electrode of the other device for insuring the flow of current through said other device, said last mentioned means comprising a transformer having one of its windings connected in series with one device and another winding connected in series with the other device, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of turns being connected in series with the anode of the device that conducts the greater current, and the winding having the larger number of turns being connected in series with the anode of the device that conducts the smaller current whereby current fiow through one device will initiate the operation of the other device and whereby currents through said devices are divided in accordance with their ratings.

2.-In a circuit arrangement of the class described, a source of direct current, a pair of different types of arc-discharge devices having certain corresponding electrodes thereof connected in parallel to said source, means for applying a control voltage to one of said devices for initiating the fiow of current therethrough, and means responsive to the flow of current throughsaid one device for increasing the voltage applied to the more positive electrode of the other device for insuring the flow or" current through the other device, said last-mentioned means comprising a transformer having one of its windings connected in series with one device and another winding connected in series with the other device, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of turns being connected in series with the anode of the device that conducts the greater current, and the winding having the larger number of turns being connected in series with the anode of the device that conducts the smaller current whereby current flow .through one device initiates the operation of the other device and whereby currents through said devicesare divided in accordance with their rat- 3. In a circuit arrangement of the class described, a source of current, parallel branches connected across said source, each branch having arc-discharge devices of unequal current handling ratings connected in series therein, means for initiating current flow through the devices in one of said branches, and means responsive to current flow through the devices in said one branch for insuring current flow through the devices in the other branch, said last-mentioned means comprising a transformer having one of its Windings connected in series with one device and another winding connected in series with the other device, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of turns being connected in series with the anode of the device that conducts the greater current, and the winding having the larger number of turns being connected in series with the anode of the device that conducts the smaller current whereby current flow through one device initiates the operation of the other device and whereby currents through said devices are divided in accordance with their ratings 4. In a circuit arrangement of the class described, a source of current, parallel branches connected across said source, each branch having arc-disharge devices of unequal current handling ratings connected in series therein, means for initiating in succession the current flow through the respective arc-discharge devices in one of said branches, and means responsive to current flow in said one branch for insuring current fiow through the arc-discharge devices in the other branches, said last-mentioned means comprising a transformer having one of its windings connected in series with one device and another winding connected in series with the other device, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of turnsbeing connected in series with the anode of the device that conducts the greater current, and the winding having the larger number of turns being connected in series with the anode of the device that conducts the smaller current whereby current flow through one device initiates the operation of the other device and whereby currents through said devices are divided in accordance with their ratings.

5. In a circuit arrangement of the class described, a source of current, a pair of main branches connected in parallel across said source, each main branch comprising sub-branches connected in parallel, each sub-branch having included therein an arc discharge device, said are discharge device having unequal current handling ratings, means for starting current flow from said source through an arc discharge device of one sub-branch, means responsive to said current flow for eifecting current flow in all of the subbranches of the main branch associated therewith, said effecting means comprising a transformer having one of its windings connected in series with one device and another winding connected in series with the other device, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having 10 the smaller number of turns being connected in series with the anode of the device that conducts the greater current, and the winding having the larger number of turns being connected in series with the anode of the device that conducts the smaller current 'whereby current flow through one device initiates the operation of the other device and whereby currents through said devices are divided in accordance with their ratings and means responsive to the current flow in said lastmentioned main branch for insuring current flow through the arc discharge devices in all of the sub-branches of the other main branch said insuring means comprising a transformer and having one of its windings connected in series with one device and another winding connected in series with the other device, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of turns being connected in series with the anode of the device that conducts the greater current, and the Winding having the larger number of turns being connected in series with the anode of the device that conducts the smaller current whereby current flow through one device initiates the operation of the other device and whereby currents through said devices are divided in accordance with their ratings.

6. In a circuit arrangement of the class described, a source of direct current, a pair of arcdischarge devices of unequal current handling ratings, each device comprising at least a primary electrode and a secondary electrode surrounded by an ionizable atmosphere, a primary electrode of each device being connected in multiple to one terminal of said source and the secondary electrode of each device being connected in multiple to the other terminal of said source, means for applying a control voltage for initiating the operation of at least one of said devices, and a closely coupled transformer having a pair of windings, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of turns being connected in series with the secondary electrode of the device that conducts the greater current, and the winding having the larger number of turns, being connected in series with the secondary electrode of the device that conducts the smaller current, and the windings being properly poled, whereby current flow through one device will initiate the operation of the other device, and whereby the currents through said devices are divided in accordance with their ratings.

'7. In a circuit arrangement of the class described, a source of direct current, a pair of electrostatically controlled arc-discharge devices of unequal current handling ratings, each device comprising a cathode, an anode and a grid surrounded by an ionizable atmosphere, the anodes and the cathodes being respectively connected in multiple to the terminals of said source, means for applying control voltages to said devices for initiating the operation of at least one device, and a closely coupled transformer having a pair of windings, the turns ratio of said windings being equal to the desired current ratio of said devices, the winding having the smaller number of tu being connected in series with the anode of th device that conducts the greater current, and th winding having the larger number of turns bein connected in series with the anode of th device that conducts the smaller current whereb current flow through one device will initiate th operation of the other device, and whereby th .121 12 :eumnts through said devlces "fire divided in '&0- Number Name fiafi cordance with their'ratings. 1,902,460 Mittag Mar. 21,1933 HAROLD GOLDBERG. 2,005,875 Silverman June 25, 1935 -EDWARD-F.-DE 'MERS. 2,123,018 Morrison Ju1y 5, 1938 5 2,140,736 Demontvignier Dec. 20,1938 REFERENCES CITED 2,326,550 -Mittag Aug. 10, 1943 The followin references are of "record in the 2,343,555 Mathes May file of this pwt ent: 2,359,747 Carleton Oct. 10, 1944 "2,365,450 Bliss Dec. 19, 1944 UNITED STATES PATENTS 10 2,400,456 'Haine May 14, 1 948 Number Name Date 1,902,227 Garretson Mar. 21, 1933 

